Membrane template synthesis of microtube engines

ABSTRACT

Methods, structures, devices and systems are disclosed for fabrication of microtube engines using membrane template electrodeposition. Such nanomotors operate based on bubble-induced propulsion in biological fluids and salt-rich environments. In one aspect, fabricating microengines includes depositing a polymer layer on a membrane template, depositing a conductive metal layer on the polymer layer, and dissolving the membrane template to release the multilayer microtubes.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent document claims the priority of U.S. provisional applicationNo. 61/492,782 entitled “MEMBRANE TEMPLATE SYNTHESIS OF CATALYTICMICROTUBE ENGINES” filed on Jun. 2, 2011, which is incorporated byreference as part of this document.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant CBET 0853375awarded by the National Science Foundation. The government has certainrights in the invention.

TECHNICAL FIELD

This patent document relates to systems, devices, and processes withrespect to nano or microstructures and machines based on suchstructures.

BACKGROUND

Nanotechnology provides techniques or processes for fabricatingstructures, devices, and systems with features at a molecular or atomicscale, e.g., structures in a range of one to hundreds of nanometers insome applications. For example, nano-scale devices can be configured tosizes similar to some large molecules, e.g., biomolecules such asenzymes. Nano-sized materials used to create a nanostructure,nanodevice, or a nanosystem that can exhibit various unique propertiesthat are not present in the same materials scaled at larger dimensionsand such unique properties can be exploited for a wide range ofapplications.

SUMMARY

Techniques, systems, and devices are disclosed for fabricating andimplementing self-propelling nanostructures and microstructures usingmembrane template electrodeposition.

In one aspect, a method is provided for fabricating one or moremicrotubes to include depositing a first layer on a template that hasone or more holes to form a tube of the first layer in each hole;depositing a second layer over the first layer inside each hole of thetemplate to form a bilayer microtube formed of the first and secondlayers inside each hole; and separating the template from each bilayermicrotube.

In another aspect, a method is provided for fabricating one or moremicrotube to include depositing a first layer on a template that has oneor more holes to form a tube of the first layer in each hole; depositingan intermediate second layer over the first layer inside each hole;depositing a third layer over the intermediate second layer inside eachhold to form a trilayer microtube formed of the first, intermediatesecond, and third layers inside each hole; and separating the templatefrom each trilayer microtube.

The subject matter described in this patent document can be implementedin specific ways that provide one or more of the following features. Forexample, the disclosed technology includes techniques to fabricateself-propelled chemically-powered catalytic nano/micromotors that canpropel by bubble-induced propulsion in a fuel fluid including biologicalfluids and salt-rich environments. For example, the exemplary membranetemplate electrodeposition techniques can be implemented to mass produceconical shape microtubes (e.g., with yields of millions of microtubes in˜30 min). In addition, for example, fabrication of trilayer microtubes(e.g., with polymer, magnetic and catalytic layers) by means of thedisclosed subject matter can provide multifunctional microtubes, whichcan facilitate motion control in various applications. For example,using the disclosed methods, microtubes of both polymer and metal layerscan be fabricated at a high yield with scalable size in both diameterand length. For example, the speed of exemplary catalytic microenginesproduced using the exemplary technique can be increased in hydrogenperoxide and can be faster than microengines produced by othertechniques. The exemplary catalytic microengines produced using thedisclosed techniques can also move rapidly in a very low hydrogenperoxide level (e.g., down to 0.2% concentration). The exemplarymicrotubes exhibit excellent propulsion characteristics in diversebiological fluids and can be used in diverse biomedical applications,e.g., lab-on-chip diagnostics, cell sorting, target isolation, targeteddrug delivery, and microsurgery. For example, in some implementations,the nano/micromotors of the disclosed technology can be engineered asimmuno-nano/microscale machines that can isolate cells and/or targetmolecules from complex samples in vitro in a variety of biomedicalapplications, e.g., including drug delivery to biosensing. Also, forexample, the described chemically-powered nanomotors and micromotors canbe configured to move and pick-up/transport payloads in physiologicalconditions, e.g., within environments having high ionic strength, suchas biological fluids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic diagram of an exemplary membrane-templateelectrodeposition fabrication process of microtube engines.

FIG. 1B shows scanning electron microscopy (SEM) images of exemplaryPANI/Pt bilayer microtube engines.

FIGS. 2A and 2B show images of trajectories and motion of exemplarytubular catalytic PANI/Pt microengines.

FIG. 3 shows a data plot demonstrating the dependence of an exemplarymicroengine's speed upon the hydrogen peroxide fuel concentration.

FIGS. 4A-4C show images of an energy-dispersive X-ray (EDX) mappinganalysis of exemplary bilayer PANI/Pt microtubes.

FIG. 5 shows a data plot demonstrating the influence of sodium cholateconcentration in the fluid on the speed of the exemplary catalyticmicroengines.

FIG. 6 shows images of the propulsion motion of exemplary catalyticbilayer microengines in biological media.

FIG. 7 shows an image demonstrating magnetic motion control of anexemplary catalytic PANI/Ni/Pt trilayer microengine.

FIG. 8 shows time lapse images demonstrating the motion of an exemplarycatalytic PANI/Ni/Pt trilayer microengine under a magnetic field.

FIG. 9A shows a schematic illustration of an exemplary acid-drivenPANI/Zn microengine in an acidic fluid environment.

FIG. 9B shows an SEM image of exemplary PANI/Zn microtube engines.

FIG. 10 shows time lapse images demonstrating the motion of an exemplaryacid-powered PANI/Zn bilayer microengine.

FIG. 11 shows a data plot showing the pH dependence of the speed ofexemplary PANI/Zn microengines in solutions of different acidicconcentrations.

FIGS. 12A-12D show time lapse images showing the propulsion and cargomanipulation of an exemplary Ni/Ti/PANI/Zn microengine.

FIG. 13A shows a schematic diagram of an exemplary membrane-templateelectrodeposition fabrication process of functionalized microtubeengines.

FIG. 13B shows SEM images of exemplary PEDOT/Pt microtube engines.

FIG. 13C shows an illustrative picture demonstrating the oil removalfunctionality of SAM-modified catalytic microtube engines.

FIGS. 14A and 14B show images of exemplary SAM-modified and unmodifiedmicroengines in the presence of oil droplets within a fluid.

FIG. 15A shows a set of images showing the capture and transport ofmultiple small olive oil droplets by an exemplary dodecanethiol-modifiedAu/Ni/PEDOT/Pt microengine.

FIG. 15B shows a data plot displaying the dependence of speed upon thenumber of transported oil droplets by exemplary SAM-modifiedmicroengines.

FIGS. 16A and 16B show images of exemplary hexanethiol-modifiedmicroengines with different head functional groups interacting with oildroplets.

FIG. 17 shows images showing the effect of thiol length chain onSAM-modified microengine-oil interaction.

FIG. 18 shows time lapse images of an exemplary dodecanethiol-modifiedmicroengine in an oil-contaminated water sample.

FIG. 19A shows SEM images of polymer-based template growth of exemplarybilayer microtubes.

FIG. 19B shows images of an EDX analysis of exemplary polymer-basedbilayer microtubes.

FIG. 20 shows a data plot of the absolute and relative speeds of theexemplary polymer-based bilayer microtubes.

FIG. 21 shows SEM images of PEDOT-based bilayer microtube prepared underdifferent conditions.

FIGS. 22A and 22B show images of exemplary PEDOT/Pt microtubes withopening diameters of less than 800 nm.

FIG. 23 shows images of the propulsion of exemplary PEDOT/Ptmicroengines in a fuel solution at a physiological temperature.

FIG. 24 shows images of propulsion of an exemplary PANI/Pt microengineat room temperature in a fuel solution with hydrazine.

FIGS. 25A and 25B show images of the propulsion of exemplary PPy-basedtubular microengine using silver and a platinum-nickel alloy innerlayer.

FIG. 26A shows an SEM image of an exemplary PPy/Au_(rough) bilayermicrotube engine.

FIG. 26B shows an image of the biocatalytic propulsion of an exemplaryPPy/Au-catalase microtube engine in a fuel fluid.

FIG. 27 shows images demonstrating the propulsion of an exemplary Au/Ptbimetallic microtube engine in a fuel fluid.

FIG. 28 also shows SEM images and EDX mapping data of exemplary Au/Ptbimetallic microtube engine.

Like reference symbols and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The techniques, systems, and devices described in this patent documentcan be used for production of nanoscale or microscale structures usingthe disclosed techniques including membrane template electrodepositionthat are capable of autonomous movement and additional functionalities,e.g., including pick-up and transport of targeted payloads. For example,the engineered microstructures of the present technology can beconfigured as microtube rockets or microrockets, microtube engines ormicroengines, microtube motors or micromotors, micromachines,microtubes, and microcones.

In one aspect, the disclosed technology includes syntheticchemically-powered nanoscale motors that can self-propel in a fluidenvironment by converting energy into movement and forces. Among theseexemplary chemically-powered nanomotors, catalytic microtube engines ofthe disclosed technology can exhibit efficient bubble-induced propulsionin relevant biological fluids and salt-rich environments containing afuel substance. For example, these exemplary catalytic microtube enginescan propel in a fluid by a gas-bubble propulsion mechanism based on thefuel fluid and the shape/geometry and inner surface material of themicroengine structure. An exemplary catalytic microtube engine can bestructured to include a large opening and a small opening that are onopposite ends of the microtube, in which the microtube includes a tubebody connecting the openings and has a cross section spatially reducingin size along a longitudinal direction from the large opening to thesmall opening. The microtube engine can include a layered wall whichincludes an inner layer having a catalyst material that is reactive witha fuel fluid to produce bubbles exiting the tube from the first largeopening to propel the tube to move in the fuel fluid. Additionally, thelayered wall of the microtube engine can include an external layerformed of a material capable of being functionalized, e.g., by amolecular layer functionalized onto the external layer of the tube andstructured to attach to a target molecule. For example, the inner layercan include a surface exposed to the fuel fluid that includes a catalystmaterial, e.g., platinum (Pt).

In some implementations, a microengine based on the disclosed technologycan autonomously move by facilitating the entrance of a fuel (e.g.,hydrogen peroxide (H₂O₂)) through the small radial opening of themicroengine. Catalytically-generated gas bubbles (e.g., oxygenmicrobubbles (O₂)) can be formed and travel along the sloped transitionof the inner surface of the microengine. The oxygen microbubbles can beejected out from the large radial opening of the microengine, whichgenerates a force propelling the microengine in the fluid. For example,the fluid can be a biological sample, e.g., including biological fluids,such as, but not limited to, aqueous humour and vitreous humour, bile,blood (e.g., blood serum, blood plasma), cerebrospinal fluid,intracellular fluid (e.g., cytoplasm) and extracellular fluid (includinginterstitial fluid, transcellular fluid, plasma), digestive fluid(including gastric juice and intestinal juice), lymphatic fluid andendolymph and perilymph, mucus (including nasal drainage and phlegm),peritoneal fluid, pleural fluid, saliva, sebum (e.g., skin oil), semen,sweat, tears, urine, vaginal fluids, and bacterial lysates. Otherexemplary fluids can include non-biological fluids, such as (but notlimited to), for example, pure water, salt-containing water,sugar-containing water, juice, and oil-based fluids.

Such a microengine can be configured to include additionalfunctionalities. For example, such a microengine can be functionalizedto capture and transport substances and organisms, e.g., such asbiomolecules or living cells (e.g., among multitudes of other cells,e.g., normal cells). For example, upon encountering target cells in thebiological fluid, the functionalized surface containing ligand moleculescan recognize integrins or other biomolecules to which the ligandmolecules have an affinity on the surface of the target cells. Based onrecognition of the surface integrins of the target cell by the ligandmolecules functionalized to the microengine, such a microengine can bedesigned to allow selective pick-up and transport of the cancer cell bythe functionalized microengine over a preselected path. For example,chemically functionalized microengines can be designed to continue toexhibit efficient locomotion and a large towing force. For example,target biomolecules can include nucleic acids, lipids, carbohydrates,peptides, proteins, enzymes, hormones, antibodies, glycoproteins,glycolipids, organelles, endotoxins, viruses, and other biologicalmaterials and biomarkers. Exemplary living organisms can include cells,e.g., healthy cells, cancer cells, bacterial cells, and other types ofcells.

Components or structures of microengines can be prepared by usingvarious techniques, including, e.g., top-down photolithography, e-beamevaporation, and stress-assisted rolling of functional nanomembranes onpolymers into exemplary microtube engines. Some implementations of thesefabrication processes can create complexities in their practical utilityand can lead to expensive costs (e.g., clean-room costs). The presenttechnology provides techniques for preparing the disclosed microenginesusing a membrane-template mass production approach that simplifies thefabrication of micromachines. For example, an exemplarymembrane-template electrodeposition technique can include usingmicroporous membranes containing a large number of uniform conical pores(e.g., including a double conical pore array) to deposit polymeric andmetallic materials to form the microtubes. The exemplarymembrane-template electrodeposition method can be implemented tosynthesize the disclosed microtube engines in a manner that isinexpensive to produce high yields.

FIG. 1A shows a schematic diagram of an exemplary membrane-templateelectrodeposition process for preparation of microtube engines of thedisclosed technology. This exemplary process uses a membrane-templatemass production to produce bilayer microtube engines including apolyaniline (PANT) outer layer and platinum inner layer using porousmembranes. The exemplary process 110 is shown to assemble a porousmembrane 111 having conical pores 113 to a substrate 112. The conicalpores 113 includes an internal tubular section that has a cross sectionthat spatially reduces in size along a longitudinal direction from thelarge opening to the small opening. In the example in FIG. 1A, eachconical pore 113 includes two tubular sections where they share thecommon small opening so that the cross section of each conical pore 113begins with the first large opening from the top surface of the templateto gradually reduce in size to a smallest cross section and thengradually increase in size to the second large opening on the bottomsurface of the template. In this design, each conical pore 113 includestwo conical sections that shore the same small opening. The two conicalsections may be structured to form an asymmetrical double cone porestructure with respect to the smallest section shared by the two conicalsections. In the examples provided below, only one conical section ofthe asymmetrical double cone pore structure is used for forming thebilayer microtube engines.

Various processes can be used to form the layers for bilayer microtubeengines inside the conical pores 113. For example, the process 110 caninclude a layer of a conductive material onto one porous side of themembrane 111 to form the substrate 112 via a suitable process, e.g., asputtering process, an electron-beam evaporation process, an atomiclayer deposition (ALD) process, or a chemical vapor deposition (CVD)process. For example, the membrane 111 can include a large number ofpores on a single membrane material, e.g., the common polycarbonatemembranes have a pore density of 10⁵-6×10⁸ pores/cm², which can enablethe mass production of the microtube structures. The membrane 111 caninclude different pore size (e.g. 0.6 μm, 1 μm, 2 μm, 5 μm). In someexamples, the membrane 111 can include a cyclopore polycarbonatemembrane with a 20 μm thickness having an asymmetrical double cone porestructure with a 2 μm diameter at both openings and a 1 μm diameter as aminimum diameter internally within the pores. The substrate 112 caninclude an electrically conductive material, e.g., such as gold, silver,copper, aluminum or others. The assembled porous membrane-substrate canbe used as a working electrode.

In FIG. 1A, the exemplary membrane template electrodeposition processcan include a process 120 to deposit an outer layer 121 (e.g., of apolymer material, including polyaniline) to form single-layer structurewithin the pores 113 of the membrane 111. For example, aniline can beelectropolymerized into the pores 113 of the membrane 111, in whichpolyaniline can grow as a tube structure, e.g., due to coupling ofoxidized monomers that bind to the negatively charge wall of themembrane 111. For example, aniline monomers can polymerize on the innerwall of the membranes due to solvophobic effects and electrostaticeffects, leading to a rapid formation of a polyaniline film.

Still referring to FIG. 1A, the exemplary membrane-templateelectrodeposition process can include a process 130 to deposit an innerlayer 132 (e.g., a catalytic material including Pt) to form bilayermicrotubes 140 within the pores 113 of the membrane 111. For example, aplatinum layer can be subsequently plated along the inner surface of thepolymer layer 121 (e.g., the PANI layer) using a galvanostatic method.For example, the high conductivity property of aniline in acid conditioncan provide support for platinum deposition, leading to a formation ofthe bilayer tube structure of platinum inside the PANI layer withinmembrane pores. Subsequently, the exemplary membrane-templateelectrodeposition process can include a process to dissolve the membrane111 and release of the bilayer microtubes 140. For example, dissolutionof the membrane 111 can include the use of methylene chloride, amongother organic solvents. The resulting conical bilayer microtubestructure 140 can be preserved after the template dissolution.

In some implementations, the exemplary membrane-templateelectrodeposition process can include additional deposition processingto deposit one or more intermediate layers between the outer layer 121and inner layer 132. For example, the additional deposition processingcan be implemented subsequent to the process 120 and prior to theprocess 130 to form a trilayer structure or other multilayer structureswithin the pores 113 of the membrane 111. In some examples, a magneticmaterial layer (e.g., nickel) can be subsequently plated along the innersurface of the polymer layer 121 (e.g., the PANI layer) using agalvanostatic method. For example, by depositing an intermediatemagnetic material, the steering of the exemplary trilayer or multilayermicrotube can be controlled via magnetic steering/motion control, e.g.,by an external magnetic field.

FIG. 1B shows scanning electron microscopy (SEM) images 150 and 160 ofthe exemplary PANI/Pt bilayer microtube engines. The image 150 shows aside view of a bilayer PANI/Pt microstructure, and the image 160 shows across-view of the bilayer PANI/Pt microstructure. For example, theexemplary structure shown in FIG. 1B is an 8-μm long conical microtubewith a defined geometry including a larger outer diameter of 2 μm and asmaller outer diameter of 1.1 μm, and a larger inner opening diameter of1.5 μm and a smaller inner opening diameter of 0.5 μm, with a taperangle of 3.2° between the smaller and larger openings. The exemplaryPANI and Pt layers, shown in the cross-view image 160, include athickness of 180 nm and 80 nm, respectively. The exemplary microtubeengines can be fabricated with significantly larger openings, lengths,and layer thicknesses using the disclosed membrane-template synthesistechniques (e.g., including openings in a 5-10 μm diameter range).

Exemplary fabrication processes and implementations were performed todemonstrate the uniform and efficient production and the functionalitiesand capabilities of the disclosed microengine technology. For example,multilayer catalytic microtubes were prepared using the describedmembrane template-directed electrodeposition process.

In one example, a cyclopore polycarbonate membrane, e.g., containing 2μm diameter conical-shaped micropores, was employed as the exemplarytemplate. A 150 nm gold film was sputtered on one side of the porousmembrane to serve as working electrode. A Pt wire and an Ag/AgCl with 3M KCl were used as counter and reference electrodes, respectively. Theexemplary membrane was then assembled in a plating cell with an aluminumfoil serving as contact. For example, polyaniline was distilled beforeuse at a vapor temperature of 100° C. and a pressure of 13 mmHg, e.g.,in which the distilled aniline solution was used within 3 days. Forfabrication of exemplary PANI/Pt bilayer microtube engines, polyanilinemicrotubes were electropolymerized for 5 sec at +0.80 V from a platingsolution containing 0.1 M H₂SO₄, 0.5 M Na₂SO₄ and 0.1 M aniline.Subsequently, the inner Pt tube was deposited galvanostatically at −2 mAfor 3600 sec from a platinum plating solution. For fabrication ofexemplary PANI/Ni/Pt trilayer microtube engines, polyaniline microtubeswere deposited from a plating solution containing 0.1 M H₂SO₄, 0.5 MNa₂SO₄ and 0.1 M aniline and electropolymerized at +0.8 V for 5 sec;then a nickel layer was deposited from a nickel plating solutioncontaining 20 g/L NiCl₂.6H₂O, 515 g/L Ni(H₂NSO₃)₂.4H₂O, and 20 g/L H₃BO₃at −1.0 V (vs. Ag/AgCl) for 1 C; finally, the inner Pt tube wasdeposited galvanostatically at −2 mA for 1800 sec. For eitherconfigurations of the multilayer microtube engines, the sputtered goldlayer substrate was completely removed, e.g., by hand polishing with 3-4μm alumina slurry (e.g., which can be indicated by visual inspection ofthe membrane color). For example, an incomplete removal of the substratecan result in bubbles emerging from the smaller opening (yet withoutcompromising the performance). The membrane was then dissolved inmethylene chloride for 10 min to completely release the microtubes. Themicrotubes were collected by centrifugation at 6000 rpm for 3 min andwashed repeatedly with methylene chloride (e.g., three times), followedby ethanol and ultrapure water (e.g., 18.2 MΩcm), twice of each, with a3 min centrifugation following each wash. The exemplary collectedmicroengines were stored in nanopure water at room temperature when notin use.

The exemplary microengine fabrication method can be characterized withgood reproducibility. For example, two batches tested from differentmembranes yielded average speeds of 286 and 281 μm/s, with relativestandard deviations of 16.4 and 18.2%, respectively. The total number ofmicroengines per batch was approximately 30. The exemplary fluid thatthe fabricated microengines propelled within contained 1.0% H₂O₂ and1.6% sodium cholate. The exemplary microengines propelled continuouslyfor over 20 min in 15 μL mixed solution (e.g., until this exemplarysample solution dried up).

The exemplary template electrochemical deposition technique to fabricatemicrotube engines was carried out with a CHI 621A potentiostat (CHInstruments, Austin, Tex.). Scanning electron microscopy (SEM) imageswere obtained with a Phillips XL30 ESEM instrument, e.g., using anacceleration potential of 20 kV. Mapping analysis was investigated byOxford EDX attached to SEM instrument and operated by Inca software. Aninverted optical microscope (Nikon Instrument Inc. Ti-S/L100), coupledwith a 40× objective, a Photometrics QuantEM 512/SC camera (RoperScientific, Duluth, Ga.) and a MetaMorph 7.6 software (MolecularDevices, Sunnyvale, Calif.) were used for capturing movies, e.g., at aframe rate of 30 frames per sec. The speed of the microengines wastracked using a MetaMorph tracking module and the results werestatistically analyzed using Origin software.

For example, in order to self-propel catalytic microengines, aqueoushydrogen peroxide solutions with concentrations ranging from 0.2-5.0%were used as chemical fuels, e.g., containing 0.33-5.0% (w/v) sodiumcholate to reduce the surface tension and facilitate the enginepropulsion. For example, below 0.5% peroxide, the fraction of movingmicroengines decreased due to the lower bubble frequency (e.g., weakerbubbling thrust). For example, in addition to the exemplaryimplementations of the fabricated microengines in fluids containingaqueous hydrogen peroxide and sodium cholate solutions, exemplaryimplementations of the fabricated microengines were performed in humanserum samples from human male AB plasma and cell culture media. Forexample, these exemplary human serum implementations were carried out bymixing sequentially 5 μL microengine solution, 5 μL 10% sodium cholate,10 μL biological media and 5 μL 7.5% H₂O₂, e.g., a final solutioncorresponding to 40% of the raw samples.

FIG. 2A shows images 210 and 220 of exemplary tubular catalytic PANI/Ptmicroengines demonstrating trajectories including spiral motion (e.g.,shown in the image 210) and circular motion (e.g., shown in the image220) during a 3 sec period. The trajectories are visualized bymicrobubble tails in a fluid including a 1% H₂O₂ solution with a sodiumcholate level of 0.33% (w/v). The exemplary images 210 and 220demonstrate the substantially large propulsion power of thetemplate-prepared PANI/Pt microtube engines. The two microengines moverapidly in spiral and circular trajectories, with an average speed of120 μm/s. For example, as shown in FIG. 2A, long oxygen bubble tailswere released from the wider tubular openings, e.g., with individualmicrobubbles size of ˜2.5 μm. The exemplary scale bars shown in FIG. 2Arepresents 20 μm.

FIG. 2B shows time lapse images in 125 ms intervals demonstrating themotion of an exemplary PANI/Pt bilayer catalytic microengine in a fluidcontaining 1% hydrogen peroxide fuel (e.g., including 0.33% sodiumcholate). The exemplary scale bar shown in FIG. 2B represents 20 μm.

FIG. 3 shows a data plot 300 demonstrating the dependence of anexemplary PANI/Pt catalytic microengine's speed upon the hydrogenperoxide concentration over a 0.2 to 5.0% range, in the presence of 1.6%(w/v) sodium cholate (n=60). The data plot 300 includes an inset 310showing the speed profile over the 0.2-1.0% peroxide range. FIG. 3 alsoshows inset images 320 and 325 demonstrating the propulsion in thepresence of 0.3% hydrogen peroxide (e.g., shown in the image 320) and0.5% hydrogen peroxide (e.g., shown in the image 320).

As shown in FIG. 3, the concentration of hydrogen peroxide fuel canstrongly influences the velocity of the catalytic microengines. Forexample, the average speed of the exemplary PANI/Pt microtubular enginesincreased from 123±21.4 μm/s (e.g., 15 body lengths/s) in the 0.5%hydrogen peroxide fuel to 1410±172 μm/s (e.g., ˜180 body lengths/s) in a5% hydrogen peroxide fuel. For example, the substantially increasedspeed of the PANI/Pt microengines over the entire range of the hydrogenperoxide fuel (e.g., 0-5% H₂O₂) reflected the higher pressureexperienced by the bubbles. However, for example, using a low peroxidelevel (e.g., below 5% H₂O₂), the exemplary template bilayer microenginesdisplayed a much faster speed than microengines fabricated using othertechniques, e.g., microengines produced by rolled-up processes. Forexample, the fastest observed speed of the microengines exceeded 3 mm/s(e.g., more than 375 body lengths/s) in connection to 10% H₂O₂ and 1.6%sodium cholate. Exemplary implementations also show that suchmicroengines can move in very low peroxide levels, e.g., as shown in theinset data plot 310 of FIG. 3. The inset 310 demonstrates the dependenceof the microengine speed upon the hydrogen peroxide concentration overthe 0.2-1.0% range. For example, well defined propulsion can be observedover this exemplary range of low peroxide concentrations, e.g., withspeeds ranging between 25 and 285 μm/s. For example, a typical microtubemovement of ˜25 μm/s (e.g., over 3 body lengths/s) in a 0.2% H₂O₂solution. In contrast, for example, the lowest peroxide level forpropelling catalytic rolled-up microengines may include a 1.5% H₂O₂solution, in which the maximum propulsion speed exhibited was a lowspeed of 1 body length/s.

Also, for example, the radius and frequency of the generated oxygenbubbles can be influenced by the level of peroxide fuel. The exemplaryinset images 320 and 325 in FIG. 3 illustrate the representative bubblesize of microtubular engines. For example, the bubble frequency wasshown to increase greatly (e.g., from 16 Hz to 60 Hz) upon raising theperoxide level from 0.3 to 0.5%, while the bubble size was shown todecrease from 2.6 μm to 2.0 μm, respectively. For example, the largerbubbles with a lower frequency were associated with a lower speed (e.g.,40 μm/s in 0.3% hydrogen peroxide, as compared to 120 μm/s in 0.5%peroxide). It was also observed in the exemplary implementations thatthe average moving steps were close to the bubble radius. For example,in 0.5% hydrogen peroxide, in which the bubble radius was shown to be ˜2μm with a frequency of ˜60 Hz, the microengine speed was shown to be 122μm/s, e.g., which nearly corresponds to the product of the bubble andfrequency, indicating that the drag forces on the engines and bubblesare comparable.

FIGS. 4A-4C show images of an energy-dispersive X-ray (EDX) mappinganalysis of exemplary bilayer PANI/Pt microtubes. FIG. 4A shows a SEMimage 410 of a bilayer microtube; FIG. 4B shows an image 420 of an EDXmapping result of the distribution of carbon; and FIG. 4C shows an image430 of an EDX mapping result of the distribution of platinum. Thepresence of both platinum and carbon (from PANI) within the conicalmicrotube shown in FIGS. 4B and 4C confirms the bilayer content.

The influence of the exemplary surfactant concentration (e.g., sodiumcholate in these exemplary implementations) upon the speed of catalyticmicroengines was also examined. FIG. 5 shows a data plot 500demonstrating the influence of sodium cholate concentration on the speedof the catalytic microengines in the presence of 1% and 2% hydrogenperoxide. For example, increased speed was observed upon raising thesurfactant concentration over a 0-5% range. For example, using a 1% H₂O₂solution containing 0.33% sodium cholate, the exemplary microenginesexhibited an average speed of 120 μm/s. The speed of the exemplarymicroengines increased to around 300 μm/s in 1.6% sodium cholate, and to520 μm/s in 5% sodium cholate. For example, in the presence of the 2%H₂O₂ solution (along with 5% sodium cholate), the exemplary microenginesmoved at 1.0 mm/s. These exemplary changes reflect lower surface tensionand reduced bubble size at higher surfactant levels.

Conventional catalytic nanowire motors operate only in lowionic-strength aqueous solutions and hence cannot be applied torealistic biological environments. For example, the nano/microengines ofthe disclosed technology can address this ionic-strength limitation andcan expand the scope of nanomotors to salt-rich environments. Forexample, FIG. 6 illustrates the movement of the template-preparedmicroengine in cell culture media and human serum, respectively.

FIG. 6 shows an image 610 of the motion of an exemplary bilayercatalytic microengine in cell culture media with 1.5% H₂O₂, 2% sodiumcholate. FIG. 6 also shows an image 620 of the motion of an exemplarybilayer catalytic microengine in human serum with 1.5% H₂O₂, 2% sodiumcholate. For example, the exemplary microengines propelled efficientlyin these biological media, e.g., with high average speeds of 150 μm/s incell culture (as shown in the image 610) and 95 μm/s in serum (as shownin the image 610). For example, the decrease of speed, as compared tomore than ˜300 μm/s in water (under similar conditions), can beattributed to viscosity effects (e.g., ˜1.5 mPa·s for human serum).Also, for example, a slower speed of ˜200 μm/s was observed in a highionic aqueous solution (e.g., 3 M NaCl, with a viscosity of −1.2 mPa·s).

For example, if the microengine structures were configured as a cylindermicrorod (e.g., such as a 8 μm in length and 2 μm diameter at bothopenings), moving at the speed of 1400 μm/s, an estimated drag force canbe determined by using the Stokes' drag theory equation:

$\begin{matrix}{F_{d} = \frac{2\; \pi \; \mu \; {LU}}{{\ln \left( {L/a} \right)} - {1\text{/}2}}} & (1)\end{matrix}$

where F_(d) is the fluid resistance, U is the speed in the microengine,g is the fluid dynamic viscosity, and L and a are the length and radiusof the microtube, respectively. The estimated drag force of thisexemplary cylinder microrod microengine can be determined to be 45 pN,which would be sufficient for transporting large cargos such as cells.It is noted that the fluid may not freely flow through such a cylindermicrorod microengine because of the oxygen bubbles in the fluid flowpath.

The results of the exemplary implementations demonstrated that thedisclosed membrane template electrodeposition fabrication techniques canmass produce microengines of a comparatively small size. For example,the disclosed methods can be used to synthesize microengines withvarying diameters and lengths (e.g., including diameters in a range of1-3 μm and lengths in a range of 4-20 μm). Additionally, the results ofthe exemplary implementations demonstrated that the exemplary catalyticmicroengines fabricated by the disclosed membrane templateelectrodeposition techniques can move at high speeds (e.g., >350 bodylengths/s) and require very low fuel concentrations in a fluid (e.g.,0.2% hydrogen peroxide concentration). The results of the exemplaryimplementations demonstrated that the exemplary catalytic microenginescan propel well in diverse biological fluids, and thus be utilized indiverse biomedical applications (e.g., lab-on-a-chip diagnostics, cellsorting, target isolation, microsurgery, and drug delivery, among otherapplications).

Additional modifications to the structure (e.g., including themultilayer materials and/or geometry of structure) can be performed toconfigure the catalytic microengines for a variety of diverseapplications. For example, the disclosed fabrication techniques can beemployed to plate different outer or inner layer materials (e.g.,polymers, metals, etc.) or additional intermediate layer(s) materials,e.g., which can be used to control the steering of thenano/microstructures. While the exemplary bilayer catalytic microengines(e.g., such as those shown in FIG. 2A) move randomly, it is alsopossible to guide them magnetically. For example, trilayer microenginescan be produced using the exemplary process described and illustrated inFIG. 1A by depositing an intermediate ferromagnetic layer between theexemplary outer PANI layer 121 and the exemplary inner Pt layer 132(e.g., such as a Ni layer, to form a trilayer PANI/Ni/Pt microengine).For example, by depositing an intermediate ferromagnetic (e.g., such asa Ni layer), the steering of an exemplary microengine can be controlledvia magnetic means. FIG. 7 shows an image 700 demonstrating magneticmotion control of an exemplary PANI/Ni/Pt trilayer microengine. Theimage 700 includes an inset schematic showing the outer PANI layer 706,the intermediate nickel layer 707, and the internal platinum layer 708.The exemplary arrow in the image 700 represents the magnetic field (B)direction.

FIG. 8 shows time lapse images in 200 ms intervals demonstrating themotion of an exemplary PANI/Ni/Pt trilayer catalytic microengine underthe magnetic field in the presence of 1% H₂O₂ solution. A decrease inthe speed of the exemplary trilayer microengine was demonstrated in thisexample, as compared to the exemplary bilayer microengines. For example,the magnetically-guided directional motion of the exemplary PANI/Ni/Ptmicroengine showed a decrease in speed to 80 μm/s, as compared to the120 μm/s of the bilayer PANI/Pt microtubes (under the same conditions),e.g., reflecting the smaller opening diameter associated with the Nilayer. The exemplary scale bar shown in FIG. 8 represents 20 μm.

In another aspect, the disclosed technology can include exemplarynano/microtube structures can be configured to propel in a fluid by agas-bubble propulsion mechanism based on the chemical reactions of innersurface material of the nano/microengine structure with acidic speciesin the fluid.

Exemplary zinc-based microtube engines (e.g., PANI/Zn bilayermicrotubes) can be fabricated using the present membrane-templateelectrodeposition fabrication techniques to move by hydrogen-bubblepropulsion in strongly acidic fluid environment. An exemplaryacid-driven microtube engine can be structured to include a largeopening and a small opening that are on opposite ends of the microtube,in which the microtube includes a tube body connecting the openings andhas a cross section spatially reducing in size along a longitudinaldirection from the large opening to the small opening. The microtubeengine can include a layered wall in which an inner layer can include achemically-reactive material (e.g., zinc (Zn)) exposed to the acidicfluid. For example, the PANI/Zn bilayer microtube engines can undergoeffective autonomous motion in the acidic fluid environment without anyadditional chemical fuel. The propulsion in the acidic fluid can bedriven by continuous thrust of hydrogen bubbles generated by thespontaneous redox reactions occurring at the inner layer surface (e.g.,the inner Zn layer). For example, when the exemplary PANI/Zn bilayermicroengines are immersed in a strongly acidic medium, a spontaneousredox reaction, e.g., involving the Zn oxidation along with generationof hydrogen bubbles, occurs on their inner Zn surface:

Zn(s)+2H⁺(aq)→Zn²⁺(aq)+H₂(g) E°=0.76 V  (2)

leading to rapid propulsion that can exceed 100 body lengths/s. Zinc hasa more negative redox potential than hydrogen and thus promotes hydrogengas evolution. For example, Zn is a biocompatible ‘green’ nutrient traceelement and thus represents an attractive material for the microengineinner layer. Other materials that can be employed as the inner layermaterial include metals, e.g., iron (Fe), aluminum (Al), cobalt (Co),tin (Sn), or lead (Pb). Such metals have a more negative redox potentialthan hydrogen and potentially can lead to similar hydrogen evolutionlike Zn.

Strongly acidic environments can be found everywhere in our life, fromdiverse industrial processes to our own human stomach. The autonomousmovement of the disclosed microengines in such acidic media can thus beapplied in diverse biomedical and industrial applications. For example,the exemplary acid-driven PANI/Zn microengines can also serve as anattractive platform for sensitive pH measurements in acidicenvironments.

Exemplary fabrication processes and implementations were performed todemonstrate the uniform and efficient production and the functionalitiesand capabilities of the disclosed microengine technology. For example,multilayer acid-driven microtubes were prepared using the describedmembrane template-directed electrodeposition process.

For example, PANI/Zn bilayer microtubes was fabricated within anexemplary cyclopore polycarbonate template e.g., containing 2 μm and 5μm diameter conical-shaped micropores, which was employed as theexemplary template. A 75 nm gold film was sputtered on one side of theporous membrane to serve as working electrode. A Pt wire and an Ag/AgClwith 3 M KCl were used as counter and reference electrodes,respectively. The exemplary membrane was then assembled in a platingcell with an aluminum foil serving as contact. For example, polyanilinewas distilled before use at a vapor temperature of 100° C. and apressure of 13 mmHg, e.g., in which the distilled aniline solution wasused within 3 days. For fabrication of exemplary PANI/Zn bilayermicrotube engines, polyaniline microtubes were electropolymerized at+0.80 V for 0.02 C (and 0.1 C for the exemplary 5 μm pore sizes) from aplating solution containing 0.1 M H₂SO₄, 0.5 M Na₂SO₄ and 0.1 M aniline.Subsequently, a zinc layer has been deposited galvanostatically at −6 mAfor 1800 sec (and 2400 sec for the exemplary 5 μm pore sizes) within thePANI layer using an 80 g L⁻¹ ZnSO₄/20 g L⁻¹ H₃BO₃ solution (buffered topH=2.5 with sulfuric acid). For example, the sputtered gold layersubstrate was completely removed, e.g., by hand polishing with 3-4 μmalumina slurry (e.g., which can be indicated by visual inspection of themembrane color). The membrane was then dissolved in methylene chlorideor chloroform for 10 min to completely release the microtubes. Themicrotubes were collected by centrifugation at 6000 rpm for 3 min andwashed repeatedly with methylene chloride or chloroform (e.g., threetimes), followed by ethanol and ultrapure water (e.g., 18.2 MΩcm), twiceof each, with a 3 min centrifugation at 6000 rpm following each wash.The exemplary collected microengines were stored in nanopure water atroom temperature when not in use. For example, to achieve the magneticdirectional control of the microtube engines, the exemplary microtubesolution was evaporated onto glass slides before the sequential E-beamdeposition of 10 nm Ti (adhesion layer), 20 nm Ni (magnetic layer), overthe microtubes. Implementations of the exemplary microtube engines wereperformed in human serum samples, e.g., from human male AB plasma(Sigma-Aldrich, St. Louis, Mo.), which were carried out by mixingsequentially 5 μL of the microtube solution with 5 μL 5% Triton X-100, 5μL biological media and 5 μL 2 M HCl, leading to a final solutioncorresponding to 25% of the raw samples.

FIG. 9A shows a schematic illustration of an acid-driven PANI/Znmicroengine in an acidic fluid environment. FIG. 9B shows an SEM imageof the top view of two PANI/Zn microtubes (e.g., preparedelectrochemically using a membrane with 2 μm diameter pores). Forexample, the membrane template electrochemically produced microengineswere configured to be ˜10 μm long, having a front inner opening diameterof ˜350 nm and a rear outer diameter of ˜1.2 μm. These exemplarymicrotube engines included a ˜150 nm thick PANI outer layer and a ˜300nm Zn inner layer. For example, the presence of carbon and zinc in theresulting bilayer microtubes was confirmed by EDX mapping analysis.

FIG. 10 shows time lapse images in 120 ms intervals over a 360 ms perioddemonstrating the propulsion of an exemplary acid-powered PANI/Znbilayer microengine (e.g., having 2 μm diameter at the larger opening)at 0, 120, 240, 360 ms, respectively. For example, the fluid mediumincluded a 1 M HCl solution at pH=0 containing 1.67% Triton X-100. Theseimages shown in FIG. 10 illustrate a tail of hydrogen microbubbles(e.g., ˜4-5 μm in diameter) generated on the Zn surface of the innerlayer and released from the rear of the exemplary microengine at a rateof 75 bubbles/sec. The exemplary microengine is self-propelled in theacidic fluid at a speed over 500 μm/s, which corresponds to a relativespeed of nearly 50 body lengths/s.

For example, the speed of the acid-driven PANI/Zn microengine was shownto be dependent on the acid concentration. FIG. 11 illustrates theinfluence of the pH and HCl concentration upon the speed of theexemplary microengines. FIG. 11 shows a data plot 1100 that exhibits thepH dependence of the speed of exemplary PANI/Zn microengines insolutions of different HCl concentrations (e.g., over a range of 0-1.6M). A 1.67% concentration of Triton X-100 was added as a surfactant inthe fluid solution. The data plot 1100 includes a data curve 1101 of theexemplary microengines with a 2 μm larger opening of the tube structureand a data curve 1102 of the exemplary microengines with a 5 μm largeropening of the tube structure. Exemplary error bars show the standarddeviations of the measured speeds.

For example, both exemplary microengines displayed their highest speedwithin the highest acid concentration examined (e.g., 1.6 M HCl,corresponding to pH −0.2). For the exemplary 2 μm microengine shown inthe data curve 1101, the speed decreased gradually from 650 μm/s (at pH−0.2) to 550 μm/s (at pH 0.0), then more rapidly to 300 μm/s (at pH0.2), and then slowly to 8 μm/s (at pH 1.0). The exemplary 5 μmmicroengine shown in the data curve 1102 displayed a similar trend,along with a higher initial speed, and slightly wider pH range. Forexample, the exemplary 5 μm microengine achieved a speed of 1050 μm/s(˜100 body lengths/s) at pH −0.2, that decays gradually to 140 μm/s atpH 0.4 and then more slowly to 10 μm/s at pH 1.3. For example, the wideroperational pH range can be associated with biomedical applications ofthe disclosed acid-driven microengines, e.g., such as movement in theextreme stomach environment of pH 0.8-2.0. The lifetime of the PANI/Znacid-driven microengines can be influenced by the rate of the Zndissolution, e.g., which may range from 10 sec to 2 min for theseexemplary configurations. For example, this can be associated with thesurrounding pH (that influences the rate of the Zn dissolution) and theamount of Zn present. For example, a prolonged movement of the exemplary5 μm microengine over 1 min in a 60 mM HCl solution (pH=1.2) at a speedof 20 μm/s was observed.

The exemplary speed-pH profiles of FIG. 11 can form the basis forsensitive motion-based pH measurements in extremely acidic environments,e.g., where common glass pH electrodes lead to a large acid error.Microengine-based pH sensing could involve measurements of the speedand/or distance traveled by the microengine, e.g., which can beanalogous to a motion-based DNA sensing techniques. Exemplarymotion-based pH sensing may be implemented in applications includingdetecting changes in the stomach acidity and remote monitoring ofetching baths in semiconductor processing. For example, changingmovement in an acid gradient (e.g., chemotaxis) can also be sensed byimplementing the exemplary acid-driven microengines.

Table 1 shows a comparison of the speed of the exemplary PANI/Znmicroengines in the presence of different acids. For example, 0.5 M HCl,0.25 M H₂SO₄ and 0.167 M H₃PO₄ were utilized along with 1.67% TritonX-100 surfactant.

TABLE 1 Acid Type Speed (μm/s) HCl 140 H₂SO₄ 80 H₃PO₄ 20

For example, while a speed of 140 μm/s was observed in 0.5 M HCl,significantly slower speeds of 80 and 20 μm/s were observed in H₂SO₄ andH₃PO₄, respectively, reflecting their decreasing acid dissociationconstants. For example, such speed variations are consistent with the pHdependence of the PANI/Zn microengine observed in FIG. 11. In contrast,for example, no efficient propulsion was observed in a 0.5 M HNO₃solution, although the zinc layer was dissolved. The lack of movement innitric acid may reflect the generation of N₂O (instead of H₂), which ismuch more soluble in water, according to:

4Zn+10HNO₃(dilute)→4Zn(NO₃)₂+N₂O+5H₂O  (3)

A magnetic layer can be incorporated into the acid-driven multilayermicroengines, e.g., such as the PANI/Pt microengines. For example, thedescribed membrane template electrodeposition fabrication techniques canfurther include implementing E-beam deposition of Ti and Ni layers onthe outer PANI surface of the exemplary PANI/Pt microengines. Forexample, the addition of the magnetic material layers can allow magneticcontrol of their directionality and cargo pick up.

Exemplary implementations of the Ni/Ti/PANI/Zn microengines (e.g.,having a 5 μm diameter at the larger opening) to demonstratemagnetically-guided movement were performed in a 0.4 M hydrochloric acidsolution. The magnetically-guided Ni/Ti/PANI/Zn microengines exhibited aspeed of ˜100 μm/s in this example. It is noted that the speed theexemplary Ni/Ti/PANI/Zn microengines was slower than that of the PANI/Znmicroengines under the same conditions (e.g., 140 μm/s exhibited by thePANI/Zn microengines), e.g., reflecting the influence of the additionalTi/Ni layers on the gas-bubble propulsion mechanism.

FIGS. 12A-12D show time lapse images that show the propulsion and cargomanipulation (e.g., load, drag, and drop) of an exemplary Ni/Ti/PANI/Znmicroengine (e.g., having a 5 μm diameter at the larger opening). Forexample, FIG. 12A shows the exemplary Ni/Ti/PANI/Zn microengineapproaching exemplary 5 μm target sphere cargo. FIG. 12B shows theexemplary Ni/Ti/PANI/Zn microengine capturing the cargo magnetically.FIG. 12C shows the exemplary Ni/Ti/PANI/Zn microengine transporting thecargo over a predetermined path. FIG. 12D shows the exemplaryNi/Ti/PANI/Zn microengine and releasing the cargo, e.g., by rapidlychanging the direction the magnetic field direction. The exemplary fluidmedium included 400 mM HCl solution containing 1.67% Triton X-100(pH=0.4). For example, during the transporting operation, the speed ofthe exemplary Ni/Ti/PANI/Zn microengine decreased from 110 μm/s to 90μm/s after capturing the polystyrene cargo, e.g., reflecting theincrease fluid drag force exerted by the larger largo. For example, adrag force of ˜5 pN can be estimated, by considering the microengine asa cylindrical nanorod as described above using Eq. (1).

Exemplary implementations to demonstrate direct locomotion of theacid-driven microengines in untreated biological environments have alsobeen performed. For example, the movement of the H₂-bubble propelledPANI/Zn microengine in acidified human serum was determined to be 92μm/s (e.g., over a 5 sec period). Thus, despite the raw biologicalmedium (e.g., such as that of the human stomach), the exemplary PANI/Znmicroengines can move rapidly, albeit at a slower speed (e.g., 92 μm/sas compared to 170 μm/s in the aqueous acid solution), e.g., whichreflects the effects of a higher viscosity of human serum on thepropulsion mechanism.

The results of the exemplary implementations demonstrated that thedisclosed membrane template electrodeposition fabrication techniques canmass produce acid-driven microengines with various sizes andconfigurations, which can move at high speeds (e.g., >100 bodylengths/s) within an acidic fluid without the presence of a fuel.Additionally, the results of the exemplary implementations demonstratedthat the exemplary acid-driven microengines can propel well in diversebiological fluids (e.g., such as fluids-found in the stomach), as wellas extreme acidic environments (e.g., such as silicon wet-etchingbaths). Thus, the disclosed microengines can be utilized in diversebiomedical and industrial applications, e.g., including motion-based pHsensing.

In another aspect, the disclosed technology can includenano/microstructures that can be functionalized to capture and transportsubstances or microorganisms. For example, the disclosednano/microstructures can be configured as a microtube engine coated witha superhydrophobic molecular layer to enable the microtube to removeoil-based substances from a particular region in a fluid environment.

Oil is a major source of ocean pollution and ground water contamination.For example, the presence of oils in wastewaters as a product of variousmanufacturing processes is common in different industries. Episodes ofmajor water pollution caused by oil spillage have resulted in therelease of millions of tons each year. For example, the 1989 ExxonValdez and 2010 Deepwater Horizon incidents spilled millions of gallonsof crude oil. The removal of oils and organic solvents from contaminatedwater is thus of considerable importance for minimizing theenvironmental impact of these pollutants. Substantial efforts have thusbeen devoted to develop effective tools towards the remediation andclean-up of oil spills. However, most of the conventional methods lackselectivity and efficiency and are not cost-effective or environmentalfriendly.

Nanomachines and micromachines of the disclosed technology can be usedto implement an oil collection method that includes capture, transportand removal of oil droplets. For example, the disclosed microtubeengines can be modified with a superhydrophobic molecular layer able toadsorb oil by means of its strong adhesion to a long chain ofself-assembled monolayers (SAMs) of alkanethiols created onto the roughouter surface (e.g., outer gold layer) of the microengine device. Insome examples, the microtube engines can include a SAM-coatedAu/Ni/PEDOT/Pt structure (e.g., the SAM layers having a polar-terminalgroup) that can exhibit continuous interaction with large oil droplets,capable of loading and transporting multiple small oil droplets. Forexample, the functionality of the exemplary SAM-coated Au/Ni/PEDOT/Ptmicroengines can be influenced based on the alkanethiol chain length,polarity and head functional group, which affect the surfacehydrophobicity upon the oil-nanomotor interaction and the propulsion ofthe microengines.

For example, surfaces with superhydrophobic properties can repel waterwhile strongly interacting with nonpolar or oily liquids, which firmlyadhere to the superhydrophobic interfaces. For example, themicro-/nano-hierarchical texture and the chemical composition can beessential for promoting the superhydrophobic properties that areeffective for oil removal, e.g. such as the surface polarity androughness that affect the oil-surface interaction. SAMs formed by thespontaneous and strong chemisorption of alkanethiols on the outersurface (e.g., gold or silver surfaces) of the exemplary microtubeengines can transform the microtubes into superhydrophobic interfaces.For example, hydrophilic surfaces can become superhydrophobic afterexposure to particular SAM assemblies, e.g., such as an octadecanethiol.For example, tailoring the ending functional group and/or the length ofan alkanethiol chain can control of the surface polarity of theSAM-modified microengines. For example, methyl-terminated SAMs canproduce hydrophobic surfaces, while hydroxyl-terminated ones providewettable surfaces.

The disclosed membrane-template electrodeposition techniques can beutilized for mass production of the exemplary superhydrophobic microtubeengines. For example, template-fabricated catalytic microtubes can beconfigured to be superhydrophobic microtube engines, e.g., in which thecatalytic microtube structure can include a polymer/Pt bilayer core(e.g., as shown in FIGS. 1A and 1B) with additional nickel and goldlayers electrodeposited on the outside of the polymer/Pt bilayer toprovide magnetic guidance and surface functionalization (e.g., withreceptors), respectively. For example, the outer surface of the outergold layer can be further modified with a SAM coating (e.g., includingan alkanethiol molecular chain). For example, the, terminal functionalgroups of the attached SAM coating can be chemically modified to attachmolecular bioreceptors, e.g., including, but not limited to, DNA probes,aptamers, antibodies and lectins.

FIG. 13A shows a schematic diagram of an exemplary membrane-templateelectrodeposition fabrication and modification process for preparationof hydrophobic microtube engines of the disclosed technology, e.g., suchas a dodecanethiol SAM-modified Au/Ni/PEDOT/Pt catalytic microengine.For example, the exemplary process includes a membrane-template massproduction technique to produce multilayer microtube engines including acore bilayer microtube having a poly(3,4-ethylenedioxythiophene) (PEDOT)outer layer and platinum inner layer using porous membranes. Also, forexample, the exemplary process includes coating techniques to formadditional outer layers, e.g., including a nickel layer and a goldlayer, over the core bilayer microtube structure, e.g., in which theexemplary Ni material layer can be used for magnetic steering and the Aulayer can be used for modification of superhydrophobic monolayercoatings. The exemplary process can include a process 1310 to assemble aporous membrane 1311 having conical pores 1313 to a substrate 1312. Forexample, the process 1310 can include sputtering a layer of a conductivematerial onto one porous side of the membrane 1311 to form the substrate1312. In some examples, the membrane 1311 can include a cycloporepolycarbonate membrane with a 20 μm thickness having an asymmetricaldouble cone pore structure with a 2 μm diameter at both openings and a 1μm diameter as a minimum diameter internally within the pores 1313. Inother examples, the membrane 1311 can include single conical pores(e.g., with a 1 μm diameter at one opening and a 2 μm diameter at otheropening). The substrate 1312 can include an electrically conductivematerial, e.g., such as gold. The assembled porous membrane-substratecan be used as a working electrode, e.g., in electroplating processes.

In FIG. 13A, the exemplary membrane-template electrodeposition processcan include a process 1320 to deposit an outer layer 1321 (e.g., of apolymer material, including PEDOT) within the pores 1313 of the membrane1311, followed by deposition of an inner layer 1322 (e.g., a catalyticmaterial including Pt) to form a bilayer core microtube structure withinthe pores 1313 of the membrane 1311. For example,3,4-ethylenedioxythiophene can be electropolymerized into the pores 1313of the membrane 1311, in which PEDOT can grow as a tube structure, e.g.,due to coupling of oxidized monomers that bind to the negatively chargewall of the membrane 1311. For example, 3,4-ethylenedioxythiophenemonomers can polymerize on the inner wall of the membranes due tosolvophobic effects and electrostatic effects, leading to a rapidformation of a PEDOT film. For example, a platinum layer can besubsequently plated along the inner surface of the polymer layer 1321(e.g., the PEDOT layer) using a galvanostatic method. For example, thehigh conductivity property of poly(3,4-ethylenedioxythiophene) in acidconditions can provide support for platinum deposition, leading to aformation of the bilayer tube structure of platinum inside the PEDOTlayer within membrane pores. Subsequently, the exemplarymembrane-template electrodeposition process 1320 can also include aprocess to dissolve the membrane 1311 and release of the core bilayermicrotubes. For example, dissolution of the membrane 1311 can includethe use of methylene chloride, among other organic solvents.

The exemplary fabrication process in FIG. 13A can include a process 1330to deposit at least one outer layer over the outer surface of the corebilayer microtubes. For example, a magnetic material layer 1331 (e.g.,such as a Ni layer) can be deposited over the outer surface of the corebilayer microtubes by e-beam vapor deposition. The exemplary Ni layercan provide the magnetic steering and navigation control functionalityto the microtube engines. For example, subsequently, an outerfunctionalization material layer 1332 (e.g., such as an Au outer layer)can be deposited over the magnetic material layer 1331 by e-beam vapordeposition. In some implementations, the selected magnetic material toserve in the magnetic material layer 1331 can also provide thefunctionality to enable further surface functionalization, e.g., such asbinding monolayers such as superhydrophobic SAMs, such that thedeposition of the outer functionalization material layer 1332 may not beimplemented.

Still referring to FIG. 13A, the exemplary fabrication process caninclude a process 1340 to form a chemical coating 1341 havingsuperhydrophobic properties, e.g., by self-assembly of long alkanethiolschains on the rough outer surface of the functionalization materiallayer 1332. For example, the chemical coating 1341 can include analkanethiol SAM layer formed on an exemplary gold layer 1332 of themicroengine surface by incubation of the microtubes in a 0.5 mMn-dodecanethiol ethanol solution for 30 min. The chemical coating 1341can include a variety of chemical structures that exhibit hydrophobicproperties, such as alkanethiols, e.g., including hexanethiol (C-6),dodecanethiol (C-12) and octadecanethiol (C-18), among others.

FIG. 13B shows an SEM image 1390 of the exemplary PEDOT/Pt microengines,including an image 1395 showing a zoomed-in view of the outer surfacezone identified in the image 1390 by the arrow. For example, the SEMimages 1390 and 1395 of the unmodified microengine indicate a roughsurface, e.g., characteristic of nitrate-doped PEDOT films.

The exemplary method shown in FIG. 13A can be implemented to produceSAM-modified microtube engines able that strongly interact with oilyliquids via adhesion and permeation onto its long alkanethiol coating.FIG. 13C shows an illustrative picture demonstrating this functionalityin an oil removal application. The relative similar dimensions ofmicroengine and oil droplets (which range from ˜1 to ˜100 μm, dependingon the emulsion composition) permit convenient real-time opticalvisualization of the oil-microengine interaction. For example, thetemplate-prepared PEDOT/Pt microtubes can propel efficiently indifferent media via the expulsion of oxygen bubbles generated from thecatalytic oxidation of hydrogen peroxide fuel along their inner Ptlayer. It is noted, for example, that the speed of the polymer/Ptmicroengines (e.g., such as the PEDOT/Pt microengines) is reduced by upto 50% after the subsequent deposition of the outer magnetic andfunctionalization layers, e.g., which may reflect partial blocking ofthe inner Pt catalytic layer. However, this exemplary reduced speed isstill sufficient for transporting large cargos. Table 2 summarizes theaverage speeds of the microengines at each stage of the formation of thedifferent layers involved in the fabrication process and oil removalapplications.

TABLE 2 Microengine type Speed (μm/s) PEDOT/Pt 420 Au/Ni/PEDOT/Pt 200SAM/Au/Ni/PEDOT/Pt 105 SAM/Au/Ni/PEDOT/Pt/few (1-5) droplets 20-50SAM/Au/Ni/PEDOT/Pt/numerous droplets 10-20

Exemplary fabrication processes and implementations were performed todemonstrate the uniform and efficient production and the functionalitiesand capabilities of the disclosed superhydrophobic microenginetechnology. For example, SAM-functionalized multilayer catalyticmicrotubes were prepared using the described membrane template-directedelectrodeposition and functionalization process. For example, exemplaryimplementations using fabricated dodecanethiol-coated Au/Ni/PEDOT/Ptmicroengines were performed to demonstrate an effective capture andtransport of oil droplets from aqueous media. For example, the influenceof the alkanethiol chain length upon the oil-nanomotor interaction andthe collection efficiency was examined in these exemplaryimplementations, e.g., by applying various chemical coatings using SAMsof different chain lengths, e.g., hexanethiol (C-6), dodecanethiol(C-12) and octadecanethiol (C-18).

For example, SAM-modified Au/Ni/PEDOT/Pt catalytic microengine werefabricated using an exemplary cyclopore polycarbonate template membrane,e.g., containing 2 μm maximum diameter conical-shaped micropores(Catalog No 7060-2511; Whatman, Maidstone, UK). For example, a 75 nmgold film was first sputtered on one side of the exemplary porousmembrane to serve as working electrode using the Denton Discovery 18.The sputter was performed at room temperature under vacuum of 5×10⁻⁶Torr, DC power 200 W and flow Ar to 3.1 mT (e.g., with rotation speed of65 and sputter time 90 s). A Pt wire and an Ag/AgCl with 3 M KCl wereused as counter and reference electrodes, respectively. The exemplaryporous membrane-substrate device was then assembled in a plating cellwith an aluminum foil serving as contact. For example, PEDOT microtubestructures were electropolymerized up to 0.1 C at +0.80 V from a platingsolution containing 15 mM EDOT monomer, 50 mM SDS and 7.5 mM KNO₃. Theinner Pt tube of the core bilayer microtube structure was depositedgalvanostatically at −2 mA for 600 s from a platinum plating solution(Platinum RTP; Technic Inc, Anaheim, Calif.). For example, the sputteredgold layer substrate was completely removed, e.g., by mechanical handpolishing with 3-4 μm alumina slurry. The exemplary bilayer coremicrotubes were collected by centrifugation at 6000 rpm for 3 min andwashed repeatedly with methylene chloride, followed by ethanol andultrapure water (e.g., 18.2 MΩcm), three times of each, with a 3 mincentrifugation following each wash. The exemplary bilayer coremicrotubes suspension was then evaporated onto glass slides prior to thesequential deposition of 10 nm Ti (e.g., acting as an adhesion layer),15 nm Ni (e.g., to form the exemplary magnetic layer 1331), and 15 nm ofAu (e.g., to form the exemplary functionalization layer 1332) over theexemplary bilayer core microtubes by using electron beam deposition.

The external surface of the Au functionalization layer 1332 wassubsequently modified by immersion in a 0.5 mM dodecanethiol in absoluteethanol, after which the resulting monolayer-modified microengine werewashed with Milli-Q water and isolated by centrifugation at 6000 rpm for4 min. The exemplary implementations were carried out at roomtemperature. Characterizations of the chemical structure effect, e.g.,length chain and terminal groups, on the speed were performed withdifferent thiols, e.g., including hexanethiol, mercaptohexanol,dodecanethiol and octadecanethiol, dissolved in ethanol.Non-functionalized (bare) microengines without the monolayer were alsoprepared, e.g., to serve as control groups in exemplary experimentalimplementations.

For example, template electrochemical deposition of microtube wascarried out with a CHI 661 D potentiostat (CH Instruments, Austin,Tex.). An inverted optical microscope (Nikon Eclipse Instrument Inc.Ti-S/L100), coupled with a 40× objective, using a Hamamatsu digitalcamera C11440 and a NIS-Elements AR 3.2 software, were used forcapturing movies at a frame rate of 20 frames per second. The speed ofthe microengines was tracked using a NIS-Elements tracking module andthe results were statistically analyzed using Origin software.

The exemplary experimental implementations included preparing anemulsion containing milliQ water/oil sample/6% sodium cholate (NaCh)(2:2:1), e.g., in order to self-propel the catalytic microengines arounddifferent oil droplets or capturing such droplets. For example, a knownvolume of this solution was spread on a glass slide and an equal volumeof a solution containing the microengines and the same volume ofhydrogen peroxide was added to get a final concentration of 0.4% NaChand 10% H₂O₂. The exemplary experimental implementations were performedusing olive and motor oils dispersed in Milli-Q water. For example,initial implementations were carried out with Nile-red stained olive oilfor improved visualization under the microscopy. However, the exemplarydye was not used in many subsequent implementations as the water-oilinterface was sufficiently distinguishable without such staining.

FIGS. 14A and 14B show images of exemplary SAM-modified and unmodifiedmicroengines in the presence of a stained olive oil droplet attached toa glass slide within a fluid. For example, the fluid included fuelconditions of 0.4% NaCh and 10% H₂O₂. FIG. 14A shows an image 1410,taken from video images in a single overlaid image, of an exemplary oilcollection method by exemplary SAM-modified catalytic microengines. Theexemplary SAM-modified catalytic microengines includeddodecanethiol-modified Au/Ni/PEDOT/Pt microtubes. As shown in FIG. 14A,the exemplary oil collection method includes a step (a) of thesuperhydrophobic catalytic microengine approaching an oil source, a step(b) of the superhydrophobic catalytic microengine making contact withthe oil source, and a step (c) of the superhydrophobic catalyticmicroengine spinning around the oil source. FIG. 14B shows an image1420, taken from video images in a single overlaid image, demonstratingthe failure of unmodified catalytic microengines to collect oil dropletsfrom the oil source. As shown in FIG. 14B, the unmodified catalyticmicroengine approached the oil source (a), made partial contact with theoil source (b), and left the oil source (c) without substantialoil-microengine interaction. The exemplary arrows in FIGS. 14A and 14Bindicate the microengine trajectory.

For example, as shown in FIG. 14A, the strong interaction between theSAM-modified microengine and an oil droplet resulted in a continuousspinning of the SAM-modified microengine around the droplet with anaccelerated speed ranging up to 200 μm/s, which occurred even after aprolonged 20 min period. These exemplary data also confirmed that theexemplary hydrogen peroxide fuel and the sodium cholate (NaCh)surfactant did not compromise its interaction with the oil droplet orthe integrity of the attached SAM. In contrast, for example, no suchinteraction was observed using the unmodified microengine (as shown inFIG. 14B), e.g., in which the bare Au/Ni/PEDOT/Pt micromotor movedrapidly past the oil droplet.

Efficient capture and transport of oil droplets was also observed whenthe exemplary SAM-modified catalytic microengines navigated incontaminated water samples containing small ‘free-floating’ oildroplets. FIG. 15A shows a set of images 1510 showing the capture andtransport of multiple small olive oil droplets by an exemplarydodecanethiol-modified Au/Ni/PEDOT/Pt microengine. The image set 1510includes video images taken after navigating in a fluid containing oildroplets and a 10% fuel solution for 5, 12, 66 and 80 s. For example, itwas observed that the longer the navigation time, the more oil dropletswere collected and confined onto the surface of the self-propelledmicromotor. For example, around 5 droplets (1.7±0.4 μm size) werecaptured and transported (as shown in the image a of the image set 1510)after 5 s of navigation, and around 40 droplets were attached to themotor surface following 80 s of navigation (as shown in the image d ofthe image set 1510). The exemplary data shown in the image set 1510demonstrate that these SAM-modified microengines provide high towingforce for transporting efficiently approximately 10-fold their volume.

For example, the increased drag force (Stokes's law) resulted in adecrease in speed of the oil-towing micromachines upon increasing thecargo size (e.g., number of captured droplets). This is illustrated in adata plot 1550 shown in FIG. 15B that displays the dependence of themicroengine speed upon the number of transported oil droplets. The dataplot 1550 also includes an illustrative schematic 1551 of an exemplarydodecanethiol-modified microengine. As shown in FIG. 15B, the speedrapidly decreased from 26 μm/s to 12 μm/s upon increasing the number ofoil droplets from 7 to 30, and then more slowly to 11 μm/s for 43 oildroplets.

The disclosed technology can include techniques to tailor the polarityof the microengine surface, e.g., which can be based on the chain lengthof an n-alkanethiol coating, and hence altering the capture andtransport properties of the engineered microengines. For example, chainlength, head groups, preparation time and other conditions (e.g.,temperature) can give rise to different SAM packing densities,configurations, and polarity.

Examination was conducted on the influence of the SAM head group andsurface polarity on the microengines-oil interaction, by comparing thebehavior of microengines coated with C6 SAM containing methyl andhydroxyl terminal groups using different time scales. FIGS. 16A and 16Bshow images of exemplary C6-SAM-modified microengines with differenthead functional groups interacting with small olive oil droplets. FIG.16A shows time lapse images 1611, 1612, and 1613 of hexanethiol-modifiedmicroengines capable of confining a payload of multiple oil droplets,corresponding to navigation times of 11, 50, and 73 sec, respectively.FIG. 16B shows time lapse images 1621, 1622, and 1623 ofmercaptohexanol-modified microengines incapable of picking up such oildroplets, corresponding to navigation times of 6.57, 6.66, and 6.71 sec,respectively. The exemplary images 1611 and 1621 include illustrativeschematics of the respective SAM-modified microengines, e.g.,hexanethiol-modified microengines in the image 1611 andmercaptohexanol-modified microengines in the image 1621. The exemplaryarrows shown in the images of FIGS. 16A and 16B indicate the directionof the microengine movement.

As shown in the FIGS. 16A and 16B, the polarity of the head functionalgroup of the attached chemical coating strongly influenced theinteraction between the exemplary modified microengines and the oildroplets. For example, as shown in FIG. 16A, small oil droplets attachedto the hexanethiol-modified microengine upon locomoting in the fluidsample. In contrast, for example, as shown in FIG. 16B, themercaptohexanol-modified microengines did not interact with the large orsmall oil droplets after rapidly contacting them, e.g., even afterprolonged locomotion of the mercaptohexanol-modified microengines withinthe fluid.

The exemplary implementations included examination of the influence ofthe alkanethiol chain length upon the oil-nanomotor interaction, e.g.,by modifying the microengines with SAMs of different alkanethiol lengths(e.g., C6, C12 and C18). FIG. 17 shows images 1710, 1720, and 1730showing the effect of thiol length chain on the SAM-modifiedmicroengine-oil droplets interaction. For example, the image 1710 showsan exemplary unmodified microengine, the image 1720 shows an exemplarymicroengine modified with hexanethiol, and the image 1730 shows anexemplary microengine modified with dodecanethiol. The exemplary imagesof FIG. 17 were taken after approximately the same time of navigation inthe fuel solution (e.g., 10% H₂O₂ with 0.4% NaCh). The exemplary arrowsindicate the direction of the microengine movement.

As shown in the FIG. 17, the alkanethiol chain length of the attachedchemical coating strongly influenced the interaction between theexemplary modified microengines and the oil droplets. For example, theC12-modified microengine exhibited strong microengine-oil interaction(e.g., shown in the image 1730 in which the C12-modified microenginespins around a large olive oil droplet), as compared to the weakerinteraction experienced by the C6-modified micromotor (e.g., shown inthe image 1720 in which the C6-modified microengine did not spin aroundthe droplet). For example, the C12-modified microengine exhibited ahigher number of captured oil droplets, e.g., as compared to the lowernumber of droplets attached to the C6-modified motor and to the absenceof captured droplets using the unmodified microengine. The exemplarydata are consistent with the surface wettability properties (e.g.,determined by contact angle measurements) of n-alkanethiols of differentlengths. For example, based on the higher hydrophobic character of longchain thiols, C18 SAM-coated microengines can offer higheroil-adsorption capabilities. However, a tradeoff between oil-absorptionability and speed may exist within the chemical coatings includinglonger alkanethiol chains. For example, the exemplary C18-SAM modifiedmicroengines exhibited slower motion which may be due to greaterblocking of their inner Pt catalytic layer in the presence of longeralkanethiols.

FIG. 18 shows time lapse images of an exemplary dodecanethiol-modifiedmicroengine collecting and transporting floating droplets motor oil in afuel-enhanced oil-contaminated water sample. For example, the fluidincluded fuel conditions of 0.4% NaCh and 10% H₂O₂, and the time lapseimages were taken from video images after 78 s of navigation. Theexemplary time lapse images of FIG. 18 show the microengine approaching(image 1810), contacting (image 1820), and carrying (image 1830) themotor oil droplets. The image 1830 in FIG. 18 includes an inset of aschematic of the exemplary dodecanethiol-modified microengine. Theexemplary arrows indicate the direction of the microengine movement. TheSAM-coated microengines displayed an ‘on the fly’ capture uponcontacting the small droplets of motor oil that were floating in thecontaminated water sample. These results exemplify the capability of thesuperhydrophobic-modified microengines for facile, rapid and highefficient collection of oils in oil-contaminated water samples.

For example, the extent of the micromotor-oil interaction and thecollection efficiency can be tuned by controlling the surfacehydrophobicity of the microengines, e.g., through the use of differentchain lengths and head functional groups. The disclosed microengines arecapable of collecting and transporting oil substances through stronginteractions between the chemically-modified superhydrophobic coatingsand large oil source, and/or by the collection and transport of multiplefree-floating small oil droplets of an oil source, e.g., present in acontaminated water sample. For example, these micromotor-oilinteractions can be utilized in the suitable final disposition of oilywastes (or other organic solvents) by collecting them in a controlledfashion within a certain spatially separated zone. For example,simultaneous parallel movement of multiple SAM-modified microengines canbe implemented to improve the efficiency of oil-removal processes. Forexample, practical large-scale oil cleaning operations can utilize thedisclosed nano/micromotors propelled by their own natural environment ordriven by an external (magnetic or electrical) control. In addition, forexample, the disclosed superhydrophobic microengines can be used for theisolation of hydrophobic molecules, e.g., drugs, and for transferringtarget analytes between liquid-liquid immiscible interfaces, or otherapplications in diverse analytical microsystems. The disclosedSAM-modified microengines can include multifunctional coatings of mixedchemically-functional layers or multiple chemically-functional layers,e.g., coupling the exemplary hydrophobic compounds with additionalfunctions (e.g., biocatalysis) into the SAMs. For example,biocatalyst-superhydrophobic microengines can be implemented in ‘captureand destroy’ operations in a variety of biomedical applications.

In another aspect, the disclosed technology includes chemically-powerednanoscale bilayer motors configured using various outer layer materialsand inner catalytic materials.

The disclosed membrane template electrosynthesis techniques can bevaried to produce a variety of polymer/metal, semiconductor/metal (e.g.TiO₂, ZnO, SiO₂, Al₂O₃), and metal/metal bilayer nanomotors andmicromotors, e.g., using on different materials and synthesisconditions, to engineer their morphology and composition to enhancetheir functionality (e.g., propulsion capabilities). For example, theexemplary template-directed electrochemical synthesis methods includethe ability to control the morphology of the structures.

For example, the influence of the composition and electropolymerizationconditions upon the propulsion of new template-prepared polymer-basedbilayer microtubular microengines is described. Exemplaryimplementations were performed to evaluate the effects of differentelectropolymerized outer layers, e.g., including polypyrrole (PPy),poly(3,4-ethylenedioxythiophene) (PEDOT), and polyaniline (PANI), aswell as various inner layer catalytic metal surfaces (e.g., Ag, Pt, Au,and Ni—Pt alloy), upon the movement of the exemplary bilayer microtubes.Microtube engines of the varying configurations of electropolymerizedouter layers and metallic and alloy inner layers were compared. Forexample, electropolymerization conditions including the monomerconcentration and medium (e.g., surfactant, electrolyte) can affect themorphology and locomotion of the fabricated microtubes. For example, themovement can be influenced by the choice of monomer structure, e.g.,which can affect the diameter and shape. The inner layer surface can beengineered with an alloyed material (e.g., Pt—Ni alloy) to providemagnetic control and catalytic fuel decomposition within one layer,e.g., simplifying the preparation of magnetically-guided microengines.Also, for example, polymer-based micromotors having an inner gold layercan produce efficient biocatalytic propulsion in low peroxide levels,e.g., in connection to an immobilized catalase enzyme.

For example, conducting polymers, such as polyaniline (PANI),polypyrrole (PPy), polythiophene (PT) andpoly(3,4-ethylenedioxythiophene) (PEDOT) can provide advantages to thefabricated bilayer microtube structures, e.g., based on their lightweight, conductivity, mechanical flexibility, unique chemical and redoxproperties, minimal structural defects, high aqueous stability, andbiocompatibility. For example, functionalized-polymer microstructurescan be used in microscale actuators, drug carriers or metabolite andgenetic biosensors. The exemplary conducting polymer materials can beemployed in the disclosed nano/microtube structures, e.g., such as theexemplary self-propelled microengines of the present technology, whichcan be prepared by the described electropolymerization techniques.Implementation of these exemplary electropolymerization techniques canoffer precise control of the deposition conditions and hence of thedimensions and morphology of the resulting fabricated microtubes. Forexample, the exact microstructure dimensions and morphologies of theresulting conducting polymers can affect the physical and chemicalproperties.

Exemplary implementations were performed to demonstrate the influence ofthe composition and electropolymerization conditions of the disclosedfabrication processes on the functionalities and capabilities of thedisclosed microengine technology.

For example, various microtube structures were prepared using theexemplary template-directed electrodeposition techniques (e.g., such asthose shown in FIG. 1A and FIG. 13A). For example, an exemplarycyclopore polycarbonate membrane, e.g., containing 2 μm diameterconical-shaped micropores, was employed as the template. For example, a75 nm gold film was first sputtered on one side of the porous membraneto serve as working electrode using the Denton Discovery 18. The sputterwas performed at room temperature under vacuum of 5×10⁻⁶ Torr, DC power200 W and flow Ar to 3.1 mT (e.g., with rotation speed of 65 and sputtertime 90 s). A Pt wire and an Ag/AgCl with 3 M KCl were used as counterand reference electrodes, respectively. The exemplary porousmembrane-substrate device was then assembled in a plating cell with analuminum foil serving as contact. PANI microtubes were prepared byimplementing the following steps. For example, aniline and pyrrole weredistilled before use at a vapor temperature of 100° C. and a pressure of13 mmHg. The nucleation and growth of conducting polymer microtubesinvolve electrostatic and solvophobic interactions between the polymersand pore wall. PANI microtubes were electropolymerized at +0.80 V for0.02 C from a plating solution containing 0.1 M H₂SO₄, 0.5 M Na₂SO₄ and0.1 M aniline, and subsequently, the inner Pt tube was depositedgalvanostatically at −2 mA for 1800 sec from a platinum plating solution(Platinum RTP; Technic Inc, Anaheim, Calif.). PEDOT microtubes wereelectropolymerized at +0.80 V for a charge of 0.06 C from a platingsolution containing 15 mM EDOT, 7.5 mM KNO₃ and 100 mM sodium dodecylsulfate (SDS), and subsequently, the inner Pt tube was depositedgalvanostatically at −2 mA for 1800 sec. For example, the smallermicrotubes (e.g., 4 μm in length) were synthesized using an exemplarycyclopore polycarbonate membrane, e.g., containing 1 μm diametermicropores. The exemplary PEDOT microtubes were electropolymerized at+0.80 V using a charge of 0.04 C while the corresponding inner platinumlayer was deposited galvanostatically at −2 mA for 1000 sec. Polypyrrole(PPy) microtubes were electropolymerized at +0.80 V for a charge of 0.8C from a plating solution containing 37 mM pyrrole and 7.5 mM KNO₃.Subsequently, for example, a metal microtube layer was plated inside thepolymer layer. For example, different metallic layers were plated indifferent polymer microtubes using the disclosed membrane templateelectrosynthesis techniques. For example, for the exemplary PPy/Ptbilayer microtubes, the inner Pt tube layer was depositedgalvanostatically at −2 mA for 1800 sec from a platinum plating solution(Platinum RTP; Technic Inc, Anaheim, Calif.). For example, for theexemplary PPy/Ag bilayer microtubes, silver was plated subsequently at−0.9 V (vs. Ag/AgCl) for a total charge of 1 C using a silver platingsolution (1025 RTU @ 4.5 Troy/gallon; Technic Inc., Anaheim, Calif.).For example, for the exemplary PPy/Pt—Ni alloy bilayer microtubes,platinum-nickel alloy was plated at −1 V for a total charge of 2 C usinga 1:1 mixed solution of a platinum solution and a nickel platingsolution containing 20 g/L NiCl₂.6H₂O, 515 g/L Ni(H₂NSO₃)₂.4H₂O, and 20g/L H₃BO₃. For example, for the exemplary PPy-Au, gold was plated at−0.9 V for 1 C from a gold plating solution (Orotemp 24 RTU RACK;Technic Inc.).

For example, the exemplary sputtered gold substrate was completelyremoved by hand polishing with 3-4 μm alumina slurry. The membrane wasthen dissolved in methylene chloride for 10 min to completely releasethe microtubes. The exemplary microtubes were collected bycentrifugation at 6000 rpm for 3 min and washed repeatedly withmethylene chloride, followed by ethanol and ultrapure water (e.g., 18.2MΩcm), three times of each, with a 3 min centrifugation following eachwash. The exemplary microtubes were stored in ultrapure water at roomtemperature when not in use.

The exemplary inner Au layer from the bilayer microtubes can befunctionalized with monolayers, e.g., such as a mixed MUA/MCH monolayer.For example, a solution of 2.5 mM MUA and 7.5 mM MCH was prepared inethanol. The exemplary microtube engines were incubated in the solutionovernight. After rinsing the tubes with ethanol for 5 min, they weretransferred to an Eppendorf vial containing a 200 μl ethanol solutionwith the coupling agents 1-Ethyl-3-[3-dimethylaminopropyl]carbodiimidehydrochloride (EDC), N-hydroxylsulfosuccinimide (Sulfo-NHS) at 0.4 M and0.1 M respectively, and the enzyme catalase (2 mg mL⁻¹). This incubationwas carried out 7 hours at 37° C. and thereafter rinsed with PBS with apH of 7.4 and SDS 0.05 wt % for 15 min at each step. Finally theexemplary microengines were washed repeatedly by centrifugation at 6000rpm for 3 min with water for three times to remove extra catalase insolution before testing.

For example, template electrochemical deposition of the exemplarymicrotubes was carried out with a CHI 661 D potentiostat (CHInstruments, Austin, Tex.). SEM images were obtained with a PhillipsXL30 ESEM instrument, e.g., using an acceleration potential of 20 kV.Mapping analysis was investigated by Oxford EDX attached to SEMinstrument and operated by Inca software. An inverted optical microscope(Nikon Instrument Inc. Ti-S/L100), coupled with a 40× objective, aPhotometrics QuantEM 512/SC camera (Roper Scientific, Duluth, Ga.) andMetaMorph 7.6 software (Molecular Devices, Sunnyvale, Calif.) were usedfor capturing movies at a frame rate of 30 frames per sec. The exemplaryspeed data of the microengines were tracked using a Metamorph trackingmodule and the results were statistically analyzed using Originsoftware. For example, in order to self-propel catalytic microtubeengines, aqueous hydrogen peroxide solutions (Sigma-Aldrich, cat. 95313)with concentrations ranging from 0.5-15.0% were used as chemical fuels,e.g., containing 2-5.0% (w/v) sodium cholate (Sigma-Aldrich, St Louis,Mo.) to reduce the surface tension.

FIG. 19A shows SEM images of polymer-based template growth of exemplaryPANI/Pt bilayer microtubes (image 1910), exemplary PPy/Pt bilayermicrotubes (image 1920), and exemplary PEDOT/Pt bilayer microtubes(image 1930). The thickness, and hence opening size, and the length ofthe polymer-based microtubes depend on the synthesis conditions. Forexample, PPy and PEDOT were electrosynthesized at +0.8 V using 7.5 mMKNO₃ as the electrolyte, and charges of 0.8 C and 0.06 C, respectively;PANI was electropolymerized using the same potential in a platingsolution containing 0.1 M H₂SO₄, 0.5 M Na₂SO₄ and 0.1 M aniline for acharge of 0.02 C. The presence of HSO₄ ⁻ anion, acidic media, and sodiumcations can provide a high PANI polymerization rate, conductivity andelectroactivity. The electropolymerization of PPy and PEDOT involvedplating solutions containing the sodium dodecyl sulfate (SDS) surfactantfor improving the monomer solubility in aqueous solution and loweringthe oxidation potential resulting in improved the opening size andsurface morphology of the microtubes. The images of FIG. 19A, involvingthese three different monomers, indicate that the disclosed templateelectrodeposition method leads to bilayer microtubes with uniformmorphology. The exemplary PANI/Pt microtubes (shown in the image 1910),PPy/Pt microtubes (shown in the image 1920) and PEDOT/Pt microtubes(shown in the image 1930) were configured to a length of ˜7 μm and werecharacterized with different front opening diameters of 0.8 μm, 0.6 μmand 1 μm, respectively. These exemplary different opening sizes can beattributed to differences in the sizes of the polymer chains,electropolymerization rate and in the packing patterns of the differentpolymers. The morphological features of the polymer films exhibited aprofound effect upon their electrochemical and electrochromicproperties. For example, comparing the formation of the PANI-, PPy-, andPEDOT-based structures, the PANI and PEDOT microtubes exhibited a largeropening and did not close up, e.g., even after a long polymerizationtime. In contrast, for example, the PPy microtubes usually have athicker layer, and form wire or fibers after long polymerization time,filling the pores completely. Exemplary EDX mapping analysis, shown inFIG. 19B, confirmed the presence of Pt, N of the exemplary PPy/Ptmicrotubes and Pt, S of the exemplary PEDOT/Pt microtubes, and thegrowth of the bilayer microtubes. FIG. 19B shows images 1950 of the EDXanalysis results of the PPy/Pt microtubes and images 1960 of the EDXanalysis results of the PEDOT/Pt microtubes.

FIG. 20 shows a data plot 2000 of the absolute and relative speeds ofthe polymer-based bilayer microtubes in a 5% H₂O₂ solution containing2.67% (w/v) sodium cholate as surfactant. The exemplary speed data ofthe data plot 2000 includes speeds of 1120, 2400 and 1700 μm/s (e.g.,corresponding to 160, 340 and 240 body lengths/s) for the exemplaryPPy/Pt, PEDOT/Pt and PANI/Pt bilayer microengines, respectively. Forexample, the exemplary differences in the speed can be attributedprimarily to the size of the opening diameter. For example, consideringthe tubular microengine as a cylinder microrod (since the fluid cannotfreely flow through microengine because of the oxygen bubbles), thefluid drag coefficient may be the same for the microengines with thesame outer diameter and different inner opening diameters; however, thelarger inner opening can be coupled with a larger catalytic surfacearea, hence facilitating the bubble evolution and leading to a fasterspeed.

Additional implementations showed that the PEDOT/Pt microenginesachieved a higher speed in a higher peroxide level (e.g., 10%), e.g., atan average speed of 3350 μm/s (e.g., ˜480 body lengths/s). For example,even further acceleration and a speed record (e.g., of ˜1400 bodylengths/s) was achieved by operating the exemplary PEDOT/Pt microenginesat a physiological temperature. The preparation of the exemplaryPEDOT-based bilayer microtubes provided more reproducible yields andconsistent batch-to-batch morphology and length, e.g., as compared tofaster growing PANI-based tubular microengines (e.g., involving a veryrapid aniline electropolymerization). The exemplary implementationsdemonstrated that the PEDOT-based microengines provided the mostfavorable preparation and propulsion performance.

FIG. 21 shows SEM images of PEDOT-based bilayer microtube prepared underdifferent conditions. Image 2101 shows an exemplary PEDOT-based bilayermicrotube fabricated using 100 mM EDOT and 100 mM SDS. Image 2102 showsan exemplary PEDOT-based bilayer microtube fabricated using 15 mM EDOTand 100 mM sodium dodecyl sulfate. Image 2103 shows an exemplaryPEDOT-based bilayer microtube fabricated using 15 mM EDOT and 2 mM SDS.Image 2104 shows an exemplary PEDOT-based bilayer microtube fabricatedusing only 15 mM EDOT. For example, a 7.5 mM KNO₃ supporting electrolytewas used during the electropolymerization process. These exemplary SEMimages show different morphological features of PEDOT/Pt tubularmicroengines, e.g., grown in the different synthesis media.

As indicated from the images of FIG. 21, the monomer concentration andthe presence of surfactant in the electropolymerization media can affectthe morphology and opening diameter of the resulting polymer microtubes.For example, the effect of the monomer concentration was examined bycomparing morphology of the microengines in electrochemical templatesystems using 15 and 100 mM EDOT (in the presence of 100 mM SDS). Thehigher monomer concentration led to less uniform, rougher tubularstructures with thicker walls (e.g., smaller opening pore), as shown inthe images 2101 versus 2102 in FIG. 21. Thus, solvophobic andelectrostatic interactions between the pore wall of membrane andreacting species can result in the preferential nucleation and growth ofPEDOT onto the pores of the membrane wall, e.g., producing microtubestructure with a thicker polymer layer. PEDOT/Pt microtubes preparedusing 15 mM EDOT solution were characterized with a smoother moreregular surface morphology, along with thinner walls. Such improvedmicrotube structures containing a wider opening pore exhibited fasterspeeds as compared with PEDOT/Pt microengines prepared in presence ofhigher monomer concentration.

For example, surfactant effects were also examined for the preparationof PEDOT/Pt microengines using the same monomer concentration. It isnoted, for example, that due to the low solubility of EDOT in water, theelectropolymerization of EDOT was usually carried out in organicsolvents (e.g., which may deteriorate the polycarbonate membrane pores).The surfactant effect upon the PEDOT/Pt microtube structures wasinvestigated using plating solutions containing different surfactantconcentrations (e.g., 2 and 100 mM) and 15 mM of the EDOT monomer. Asshown in the images 2102 and 2103 in FIG. 21, the film thicknessincreased from 150 to 100 nm upon decreasing the surfactantconcentrations from 100 to 2 mM. For example, without the surfactant,the polymerization created a non-uniform and thicker polymer layer,displayed in the image 2104 of FIG. 21, e.g., which can lead to asmaller inner diameter that greatly hinders the microengine propulsion.The results of these exemplary implementations showed that the mostfavorable surface morphology was obtained by the polymerization using alow monomer concentration in the presence of proper surfactant (e.g.,such as 100 mM concentration). For example, the improvement of thesurface quality in presence of surfactant like long-chain alkylsulfonate groups may be attributed to the decreased oxidation potentialof the monomer under the same conditions. Furthermore, for example, theaddition of surfactant has improved both the solubility of the monomerand the morphological properties of the polymer because of its dopantanion role in the polymer chain structure. Thus, the surfactant/monomerratio can also be an important parameter for controlling the surfacemorphology and physical properties in PEDOT/Pt microtubes for diverseapplications.

For example, the nature of the electrolyte was a variable that had aneffect on the yield, conductivity and morphology of the polymermicrotube structure and growth. The exemplary implementations includedcomparing the growth of exemplary PEDOT/Pt microtubes in the presence oflithium perchlorate (LiClO₄) to the growth in potassium nitrate (KNO₃),e.g., in the same aqueous media. For example, the results showed thatthe polymer growth in LiClO₄ was not well repeatable under similarpolymerization conditions yielding different lengths. In contrast, forexample, the KNO₃ electrolyte showed a substantially improved surfacemorphology, stability and thickness in the PEDOT/Pt microtubestructures. For example, charges in the KNO₃ electrolyte were consumedmore rapidly in the presence of nitrate ions than with chlorate ions,e.g., indicating the faster deposition of the PEDOT doped with nitrateions. In the presence of KNO₃, a homogenous polymer (PEDOT) surface wasdeposited onto membrane template in a short polymerization time.

Results of the exemplary implementations showed that the use ofmembranes with different pore diameters allowed for increased control onthe size of the mirotubes. For example, smaller motors can be obtained(e.g., about one half the size of microtubes of 2 μm in diameter and 7-8μm in length with a membrane with a 1 μm pore size. For example, FIG.22A shows an SEM image of a single smaller PEDOT/Pt microtube with alength of 4 μm and opening diameters of less than 800 nm (e.g., one halfthe body length of the previously described 2 μm openings-based templatebased bilayer microengines). FIG. 22B shows an SEM image of a multiplesmaller PEDOT/Pt microtube with a lengths and openings of the size shownin FIG. 22A. The results of the exemplary implementations showed thatthe smaller microengines can also achieve a very high speed. Forexample, the smaller microengine was shown to move at a speed of 325μm/s in a 4% H₂O₂ (e.g., containing 5% sodium cholate), e.g.,corresponding to a relative speed of over 80 body lengths/s.

For example, the temperature can also influence the speed of catalyticmicro/nanomotors, e.g., through its effect on the electrochemicalreactivity of hydrogen peroxide decomposition. A similar phenomenon wasobserved in the exemplary implementations of the disclosed polymer-basedmicrotubes. FIG. 23 shows images demonstrating the propulsion (over 1400body lengths/s) of exemplary PEDOT/Pt microengines in 10% H₂O₂ and 5%sodium cholate surfactant in physiological temperature of 37° C. over a40 ms period. For example, at the physiological temperature of 37° C.,the described template-based PEDOT/Pt microengines can achieve a speedof over 1400 body lengths/s (e.g., 10 mm/s in absolute speed), e.g., ascompared to around 500 body lengths/s in room temperature. For example,under the same conditions, PANI/Pt based microengines can also achieve aspeed of over 730 body lengths/s (e.g., compared to 350 body lengths/sin room temperature).

For example, in addition to increasing temperature, adding hydrazine canalso lead to a dramatic increase in the speed of disclosed catalytictubular microengines. FIG. 24 shows images of propulsion of an exemplaryPANI/Pt microengine at around 5 mm/s (e.g, corresponding to around 700body lengths/s) at room temperature in a 10% H₂O₂ solution with 0.25%hydrazine. For example, the propulsion of the exemplary platinum-basedmicroengines using hydrazine fuel alone (without any hydrogen peroxide)was also been observed. For example, the motion of these exemplaryplatinum-based microengines in a 0.1% hydrazine solution produced N₂bubbles; yet, their propulsion was observed to be short-lived (e.g., 1-2seconds) in this exemplary implementation.

The exemplary implementations also included utilizing alternativecatalytic metals other than platinum in the inner catalytic layer oftubular microengines. For example, silver can be implemented as acatalyst for hydrogen peroxide decomposition, and can therefore be usedas the inner catalytic layer of the disclosed microengines. FIG. 25Ashows an image of an exemplary PPy/Ag bilayer microengine movingefficiently at a speed of 500 μm/s in the presence of 15% hydrogenperoxide and 3% sodium cholate surfactant. For example, the silver layercan be partially dissolved in hydrogen peroxide. For example, theresults showed that the speed of the exemplary PPy/Ag microengine wasslower than of the platinum-based polymer microtubes (e.g., under sameconditions). However, the exemplary PPy/Ag bilayer microengines wereshown to still propel at fast speeds, e.g., considering the relativespeed of more than 70 body lengths/s, which is substantially greaterspeeds that speeds of conventional rolled-up platinum-basedmicroengines. The propulsion of the exemplary PPy/Ag microtubes wasobserved for over 40 min without considerable speed variations.

Magnetic guidance and control can be engineered within the catalyticmicro/nanomotors. In some examples, trilayer microengines can beconfigured with the addition of an inner nickel layer between the outerpolymer layer and the inner catalytic layer that can providemagnetically-guided propulsion. This exemplary configuration can includepreparation using a three-step electrodeposition process, e.g., such asincluding an additional electrodeposition process within the processdescribed in FIG. 1A. In other examples, a Pt/Ni alloy inner catalyticlayer that also includes magnetic properties response to an appliedmagnetic field can be employed. For example, instead of a separateplatinum inner layer and a separate nickel intermediate layer, thedisclosed membrane template electrodeposition technology can includefabrication of a Pt/Ni alloy inner layer that provides simultaneouslyboth the catalytic activity and desired magnetic navigation of thefabricated polymer/Pt—Ni microtube engine. FIG. 25B shows an image ofthe propulsion of an exemplary PPy/Pt—Ni-alloy microengine in a 10%hydrogen peroxide solution. The exemplary PPy/alloy microenginedisplayed efficient propulsion at a speed of 470 μm/s (e.g., 67 bodylengths/s).

The disclosed nano/microscale motors can also include microtube enginestructures having an inner palladium (Pd) or iridium (Ir) catalyticlayers. However, for example, it is noted that it can be difficult todeposit a defined Pd or Ir layer within the polymer layer. For example,Pd and Ir can grow on the top of the tube and block the pores, e.g.,which can result in eliminating the tubular bubble propulsion.

The disclosed technology can include nano/micromotors havingbiocatalytic layers (e.g., based on immobilized catalase, instead ofelectrocatalytic metals) for propelling the exemplary peroxide-drivennano/microscale motors. For example, a gold inner layer electrodepositedinside the outer polymeric tube layer can be used to immobilize thecatalase biocatalyst. For example, template electrodeposition of thepolymer-Au microtube can result in a very rough surface (shown in FIG.26A) for immobilizing large amounts of the enzyme. FIG. 26A shows an SEMimage of PPy/Au_(rough) bilayer micro tubular microengine. FIG. 26Bshows an image of the biocatalytic propulsion of an exemplaryPPy/Au-catalase microtube engine in a fuel fluid (e.g., 0.5% H₂O₂ and 2%sodium cholate). As illustrated in FIG. 26B, the resulting biocatalyticbilayer microengines propel favorably in the presence of a low peroxidelevel (e.g., the 0.5% H₂O₂ fuel) at a speed of 8 body lengths/s.

Also for example, the disclosed nano/microscale motors can include anouter gold layer to enable functionalization of an exemplarynano/microtube structure, which can also provide excellentbiocompatibility. FIG. 27 shows images demonstrating the propulsion ofan exemplary Au/Pt bimetallic microtube engine in a fuel fluid (e.g., 5%H₂O₂ and 2% sodium cholate). As shown in FIG. 27, the exemplary Au/Ptbimetallic microengines can propel rapidly at a speed of 1.5 mm/s in a10% H₂O₂ and 2.67% sodium cholate solution.

The exemplary Au/Pt bimetallic microtube engines couple the advantagesof an outer Au layer able to be functionalized with the effectivecatalytic activity of the inner Pt layer. The described templatesynthesis techniques of the disclosed technology can be applied toproduce these exemplary Au/Pt bilayer microengines. For example, the Auouter layer can be electrodeposited from a Au plating solutioncontaining 0.1 M NaNO₃ using DMSO as an electrolyte (e.g., withoutaddition of any other surfactants), followed by a subsequent depositionof an inner platinum layer. In some examples, the resulting gold outersurface can be relatively rough (e.g., as shown in image 2801 of FIG.28). For example, the rough surface may be attributed to the interactionof DMSO and the polycarbonate membrane. For example, DMSO may slowlydissolve the polycarbonate membranes, making the pores shrink as themicrotube is growing. This can also result in a lower yield of theseviable microtubes with a non-uniform, rough outer layer. FIG. 28 alsoshows exemplary EDX mapping data (e.g., images 2802 and 2803) thatconfirm the presence of the gold and platinum layers.

The electropolymerization conditions of the disclosed fabricationtechniques have been shown to affect the morphology and propulsionbehavior. For example, the morphology of the conducting polymermicrotube can be influenced by the nature and concentration of monomerand of the supporting electrolyte, as well as by the surfactant present,as described. For example, the disclosed nanomotors and micromotors canbe implemented in advanced sensor systems of diverse chemicals, e.g.,such as biomaterials, hydrogen peroxide or methanol fuel cell systems.Moreover, for example, the disclosed conducting polymer/metal microtubesare inexpensive and versatile and can be readily modified by the use ofa wide range of molecules that can be entrapped or used as dopants.Polymers with different functional groups (e.g., such as —OH or —COOH)can be chosen as the outer layer for different applications, hencefacilitating different surface functionalization processes. Also forexample, mixed polymers (e.g., co-polymers with more than one type offunctional group) can be selected as the outer layers material, e.g.,which can also facilitate different surface functionalization processes.These exemplary properties and advantages of the disclosed polymer-basedtubular microengines can permit their implementations in diversebiomedical and industrial applications.

In another aspect, the disclosed technology can include exemplarynano/microtube structures can be configured to propel in a fluid by agas-bubble propulsion mechanism based on the chemical reactions of innersurface material of the nano/microengine structure with basic species inthe fluid.

Exemplary aluminum-based microtube engines (e.g., PANI/Al bilayermicrotubes) can be fabricated using the present membrane-templateelectrodeposition fabrication techniques to move by hydrogen-bubblepropulsion in basic or alkaline fluid environment. An exemplarybasic-driven microtube engine can be structured to include a largeopening and a small opening that are on opposite ends of the microtube,in which the microtube includes a tube body connecting the openings andhas a cross section spatially reducing in size along a longitudinaldirection from the large opening to the small opening. The microtubeengine can include a layered wall in which an inner layer can include achemically-reactive material (e.g., aluminum (Al)) exposed to the basicfluid. For example, the polymer/aluminum bilayer microtube engines canundergo effective autonomous motion in the basic fluid environmentwithout any additional chemical fuel. The propulsion in the basic fluidcan be driven by continuous thrust of hydrogen bubbles generated by thespontaneous redox reactions occurring at the inner layer surface (e.g.,the inner Al layer). The reaction here is 2 Al(s)+2OH⁻(aq)→2 AlO₂⁻(aq)+H₂(g). For example, when the exemplary PANI/Al bilayermicroengines are immersed in a strongly basic medium, a spontaneousredox reaction, e.g., involving the Al oxidation along with generationof hydrogen bubbles, occurs on their inner Al surface. Other materialsthat can be employed as the inner layer material of the base-drivenmicrotube engines, e.g., which include metals (e.g., Na, K, Ca, Mg, orZn).

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in this patent document should not be understoodas requiring such separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this patent document.

What is claimed is:
 1. A method of fabricating one or more microtubes,comprising: depositing a first layer on a template that has one or moreholes of a desired hole geometry to form a tube of the first layer ineach hole; depositing a second layer over the first layer inside eachhole of the template to form a bilayer microtube formed of the first andsecond layers inside each hole; and separating the template from eachbilayer microtube.
 2. The method of claim 1, wherein the first layercomprises a polymer material.
 3. The method of claim 2, wherein thepolymer material comprises polyaniline (PANI).
 4. The method of claim 2,wherein the polymer material comprises polypyrrole (PPy).
 5. The methodof claim 2, wherein the polymer material comprisespoly(3,4-ethylenedioxythiophene) (PEDOT).
 6. The method of claim 1,wherein the second layer comprises a material that is reactive with afuel or is a catalyst of a fuel.
 7. The method of claim 6, wherein thematerial that is reactive with a fuel or is a catalyst of a fuelcomprises a conductive metal.
 8. The method of claim 6, wherein thematerial that is a catalyst of a fuel comprises platinum.
 9. The methodof claim 1, wherein the template comprises cyclopore polycarbonatedmembrane.
 10. The method of claim 9, wherein the cycloporepolycarbonated membrane comprises an asymmetrical, conically-shaped porestructure.
 11. The method of claim 10, wherein the asymmetricalconically-shaped pore structure comprises different cone angles.
 12. Themethod of claim 1, wherein the microtube comprises a self-propulsion.13. The method of claim 12, wherein several hundreds of microtube bodylengths per second speed is achieved.
 14. The method of claim 1, whereinthe microtube comprises a fuel based microtube.
 15. The method of claim14, wherein the fuel based microtube uses a 0.2%-30% concentrationhydrogen peroxide fuel.
 16. The method of claim 1, wherein the microtubeis fabricated to different diameters and lengths.
 17. A method offabricating one or more microtubes, comprising: depositing a first layeron a template that has one or more holes to form a tube of the firstlayer in each hole; depositing an intermediate second layer over thefirst layer inside each hole; depositing a third layer over theintermediate second layer inside each hold to form a trilayer microtubeformed of the first, intermediate second, and third layers inside eachhole; and separating the template from each trilayer microtube.
 18. Themethod of claim 17, wherein the first layer comprises a polymermaterial.
 19. The method of claim 18, wherein the polymer materialcomprises PANI.
 20. The method of claim 18, wherein the polymer materialcomprises polypyrrole (PPy).
 21. The method of claim 18, wherein thepolymer material comprises poly(3,4-ethylenedioxythiophene) (PEDOT). 22.The method of claim 17, wherein the intermediate second layer comprisesa ferromagnetic material.
 23. The method of claim 22, wherein theferromagnetic material comprises nickel, iron or cobalt.
 24. The methodof claim 17, wherein the third layer comprises a material that isreactive with a fuel or a catalyst of a fuel.
 25. The method of claim24, wherein the material that is reactive with a fuel or a catalyst of afuel comprises a conductive material.
 26. The method of claim 24,wherein the material that is reactive with a fuel or a catalyst of afuel a fuel comprises platinum.